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  掌握 Python 的 Lambda 函数.md
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  <h1 id="掌握-Python-的-Lambda-函数"><a href="#掌握-Python-的-Lambda-函数" class="headerlink" title="掌握 Python 的 Lambda 函数"></a>掌握 Python 的 Lambda 函数</h1><p>Lambda 函数是 Python 中的匿名函数。当你需要完成一件小工作时，在本地环境中使用它们可以让工作得心应手。有些人将它们简称为 lambdas，它们的语法如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">lambda arguments: expression  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p><code>lambda</code> 关键字可以用来创建一个 lambda 函数，紧跟其后的是参数列表和用冒号分割开的单个表达式。例如，<code>lambda x: 2 * x</code> 是将任何输入的数乘2，而 <code>lambda x, y: x+y</code> 是计算两个数字的和。语法十分直截了当，对吧？</p>
<p>假设您知道什么是 lambda 函数，本文旨在提供有关如何正确使用 lambda 函数的一些常规准则。</p>
<h2 id="1-不要返回任何值"><a href="#1-不要返回任何值" class="headerlink" title="1. 不要返回任何值"></a>1. 不要返回任何值</h2><p>看看语法，您可能会注意到我们在 lambda 函数中并没有返回任何内容。这都是因为 lambda 函数只能包含一个表达式。然而，使用 <code>return</code> 关键字会构成不符合规定语法的语句，如下所示：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt; integers &#x3D; [(3, -3), (2, 3), (5, 1), (-4, 4)]  </span><br><span class="line">&gt;&gt;&gt; sorted(integers, key&#x3D;lambda x: x[-1])  </span><br><span class="line">[(3, -3), (5, 1), (2, 3), (-4, 4)]  </span><br><span class="line">&gt;&gt;&gt; sorted(integers, key&#x3D;lambda x: return x[-1])  </span><br><span class="line">...   </span><br><span class="line">  File &quot;&lt;input&gt;&quot;, line 1  </span><br><span class="line">    sorted(integers, key&#x3D;lambda x: return x[-1])  </span><br><span class="line">                                   ^  </span><br><span class="line">SyntaxError: invalid syntax  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p>该错误可能是由于无法区分表达式和语句而引起的。像是包含 <code>return</code>、<code>try</code>、 <code>with</code> 以及 <code>if</code> 的语句会执行特殊动作。然而，表达式指的是那些可以被计算出一个值的表达，例如数值或其他 Python 对象。</p>
<p>通过使用 lambda 函数，单个表达式会被计算为一个值并且参与后续的计算，例如由 <code>sorted</code> 函数排序。</p>
<h2 id="2-不要忘记更好的选择"><a href="#2-不要忘记更好的选择" class="headerlink" title="2. 不要忘记更好的选择"></a>2. 不要忘记更好的选择</h2><p>lambda 函数最常见的使用场景是将它作为一些内置工具函数中 <code>key</code> 的实参，比如上面展示的 <code>sorted()</code> 和 <code>max()</code>。根据情况，我们可以使用其他替代方法。思考下面的例子：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt; integers &#x3D; [-4, 3, 7, -5, -2, 6]  </span><br><span class="line">&gt;&gt;&gt; sorted(integers, key&#x3D;lambda x: abs(x))  </span><br><span class="line">[-2, 3, -4, -5, 6, 7]  </span><br><span class="line">&gt;&gt;&gt; sorted(integers, key&#x3D;abs)  </span><br><span class="line">[-2, 3, -4, -5, 6, 7]  </span><br><span class="line">&gt;&gt;&gt; scores &#x3D; [(93, 100), (92, 99), (95, 94)]  </span><br><span class="line">&gt;&gt;&gt; max(scores, key&#x3D;lambda x: x[0] + x[1])  </span><br><span class="line">(93, 100)  </span><br><span class="line">&gt;&gt;&gt; max(scores, key&#x3D;sum)  </span><br><span class="line">(93, 100)  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p>在数据科学领域，很多人使用 pandas 库来处理数据。如下所示，我们可以使用 lambda 函数通过 <code>map()</code> 函数从现有数据中创建新数据。除了使用 lambda 函数外，我们还可以直接使用算术函数，因为 pandas 是支持的：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt; import pandas as pd  </span><br><span class="line">&gt;&gt;&gt; data &#x3D; pd.Series([1, 2, 3, 4])  </span><br><span class="line">&gt;&gt;&gt; data.map(lambda x: x + 5)  </span><br><span class="line">0    6  </span><br><span class="line">1    7  </span><br><span class="line">2    8  </span><br><span class="line">3    9  </span><br><span class="line">dtype: int64  </span><br><span class="line">&gt;&gt;&gt; data + 5  </span><br><span class="line">0    6  </span><br><span class="line">1    7  </span><br><span class="line">2    8  </span><br><span class="line">3    9  </span><br><span class="line">dtype: int64  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<h2 id="3-不要将它赋值给变量"><a href="#3-不要将它赋值给变量" class="headerlink" title="3. 不要将它赋值给变量"></a>3. 不要将它赋值给变量</h2><p>我曾见过一些人将 lambda 函数误认为是简单函数的另一种声明方式，您可能也见过有人像下面这么做：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt; doubler &#x3D; lambda x: 2 * x  </span><br><span class="line">&gt;&gt;&gt; doubler(5)  </span><br><span class="line">10  </span><br><span class="line">&gt;&gt;&gt; doubler(7)  </span><br><span class="line">14  </span><br><span class="line">&gt;&gt;&gt; type(doubler)  </span><br><span class="line">&lt;class &#39;function&#39;&gt;  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p>对 lambda 函数命名的唯一作用可能是出于教学目的，以表明 lambda 函数的确是和其他函数一样的函数——可以被调用并且具有某种功能。除此之外，我们不应该将 lambda 函数赋值给变量。</p>
<p>为 lambda 函数命名的问题在于这使得调试不那么直观。与其他的使用常规 <code>def</code> 关键字创建的函数不同，lambda 函数没有名字，这也是为什么有时它们被称为匿名函数的原因。思考下面简单的例子，找出细微的区别：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt; inversive0 &#x3D; lambda x: 1 &#x2F; x  </span><br><span class="line">&gt;&gt;&gt; inversive0(2)  </span><br><span class="line">0.5  </span><br><span class="line">&gt;&gt;&gt; inversive0(0)  </span><br><span class="line">Traceback (most recent call last):  </span><br><span class="line">  File &quot;&lt;input&gt;&quot;, line 1, in &lt;module&gt;  </span><br><span class="line">  File &quot;&lt;input&gt;&quot;, line 1, in &lt;lambda&gt;  </span><br><span class="line">ZeroDivisionError: division by zero  </span><br><span class="line">&gt;&gt;&gt; def inversive1(x):  </span><br><span class="line">...     return 1 &#x2F; x  </span><br><span class="line">...   </span><br><span class="line">&gt;&gt;&gt; inversive1(2)  </span><br><span class="line">0.5  </span><br><span class="line">&gt;&gt;&gt; inversive1(0)  </span><br><span class="line">Traceback (most recent call last):  </span><br><span class="line">  File &quot;&lt;input&gt;&quot;, line 1, in &lt;module&gt;  </span><br><span class="line">  File &quot;&lt;input&gt;&quot;, line 2, in inversive1  </span><br><span class="line">ZeroDivisionError: division by zero  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<ul>
<li><p>当您的代码存在关于 lambda 函数的问题（即 <code>inversive0</code>），<code>Traceback</code> 错误信息只会提示您 lambda 函数存在问题。</p>
</li>
<li><p>相比之下，使用正常定义的函数，<code>Traceback</code>会清晰地提示您有问题的函数（即 <code>inversive1</code>）。</p>
</li>
</ul>
<p>与此相关，如果您想多次使用 lambda 函数，最佳实践是使用通过 <code>def</code> 定义的允许使用文档字符串的常规函数。</p>
<h2 id="4-不要忘记列表推导式"><a href="#4-不要忘记列表推导式" class="headerlink" title="4. 不要忘记列表推导式"></a>4. 不要忘记列表推导式</h2><p>有些人喜欢将 lambda 函数和高阶函数一起使用，比如 <code>map</code> 或 <code>filter</code>。思考下面用法示例：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt; # 创建一个数字列表  </span><br><span class="line">&gt;&gt;&gt; numbers &#x3D; [2, 1, 3, -3]  </span><br><span class="line">&gt;&gt;&gt; # 使用带有 lambda 函数的 map 函数  </span><br><span class="line">&gt;&gt;&gt; list(map(lambda x: x * x, numbers))  </span><br><span class="line">[4, 1, 9, 9]  </span><br><span class="line">&gt;&gt;&gt; # 使用带有 lambda 函数的 filter 函数  </span><br><span class="line">&gt;&gt;&gt; list(filter(lambda x: x % 2, numbers))  </span><br><span class="line">[1, 3, -3]  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<p>我们可以使用可读性更强的列表推导式代替 lambda 函数。如下所示，我们使用列表推导式来创建相同的列表对象。如您所见，与列表推导式相比，之前将 <code>map</code> 或 <code>filter</code> 函数与 lambda 函数一起使用更麻烦。因此，在创建涉及高阶函数的列表时，应考虑使用列表推导式。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">&gt;&gt;&gt; # Use list comprehensions  </span><br><span class="line">&gt;&gt;&gt; [x * x for x in numbers]  </span><br><span class="line">[4, 1, 9, 9]  </span><br><span class="line">&gt;&gt;&gt; [x for x in numbers if x % 2]  </span><br><span class="line">[1, 3, -3]  </span><br><span class="line"></span><br></pre></td></tr></table></figure>

<h2 id="结论"><a href="#结论" class="headerlink" title="结论"></a>结论</h2><p>在本文中，我们回顾了使用 lambda 函数可能会犯的四个常见错误。通过避免这些错误，您应该能在代码中正确使用 lambda 函数。</p>
<p>使用 lambda 函数的经验准则是保持简单以及只在本地使用一次。</p>
 
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      <li>
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<script src="/js/jquery-2.0.3.min.js"></script>


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<script src="https://cdn.jsdelivr.net/npm/jquery-modal@0.9.2/jquery.modal.min.js"></script>
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<!-- ImageViewer -->

<!-- Root element of PhotoSwipe. Must have class pswp. -->
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            Don't modify these 3 pswp__item elements, data is added later on. -->
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        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
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                <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                <button class="pswp__button pswp__button--share" style="display:none" title="Share"></button>

                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
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                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.min.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script>

<!-- MathJax -->

<!-- Katex -->

<!-- busuanzi  -->


<script src="/js/busuanzi-2.3.pure.min.js"></script>


<!-- ClickLove -->

<!-- ClickBoom1 -->

<!-- ClickBoom2 -->

<!-- CodeCopy -->


<link rel="stylesheet" href="/css/clipboard.css">

<script src="https://cdn.jsdelivr.net/npm/clipboard@2/dist/clipboard.min.js"></script>
<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>


<!-- CanvasBackground -->


    
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</html>